Flour prepared from wheat has always had a special place in baking and food preparation. Fine light breads and pastries can be made from wheat flour, but not from ground preparations of other grains (which are often called by a different name, e.g., as in corn meal). Protein elasticity is crucial in determining the functional properties of wheat flours and doughs. The proteins involved are the major storage proteins, such as glutenin, which account for about 10% of the grain dry weight and form a continuous network called gluten when flour is wetted and kneaded to give dough. This network entraps carbon dioxide generated by yeasts during proving of the dough which becomes expanded to form a light porous crumb structure. Leavened bread is therefore, in essence, a protein foam which supports the other flour components, the most important of which is starch.
The ability of the wheat gluten proteins to entrap carbon dioxide to form a foam depends on a combination of two physical properties, namely, elasticity and extensibility. A precise balance of these properties is crucial, as poor quality bread can result when doughs are either insufficiently elastic (weak and too low an elastic modulus) or too elastic (overstrong and too high an elastic modulus). Gluten elasticity is also important for other applications of wheat, including the manufacture of noodles and pasta.
Wheat gluten is a complex mixture of over 50 individual proteins which are classified into two groups present in approximately equal amounts. The gliadins are monomeric proteins which interact by strong non-covalent forces (chiefly, hydrogen bonds and hydrophobic interactions) and contribute to gluten extensibility. In contrast, the glutenins consist of subunits which form high M.sub.r (approx. 1 to 10.times.10.sup.6 Da) polymers stabilized by inter-chain disulfide bonds. These polymers appear to be the major determinant of gluten elasticity, although the precise molecular basis for this is not known. However, two features which may be relevant are the number and distribution of disulfide bonds and the presence in one group of glutenin subunits of .beta.-spiral-like structures.
These proteins are called high molecular weight (HMW) subunits of glutenin, and have been studied in some detail because allelic variation in their composition is correlated with differences in the breadmaking quality of wheats (Shewry et al. (1992) J. Cereal Sci. 15: 105-120). The individual proteins vary from 627 to 827 residues in length (M.sub.r s from about 67,500 to 88,100), and consist of central repetitive domains (481 to 696 residues) flanked by non-repetitive N-terminal (84 to 104 residues) and C-terminal (42 residue) domains. The repetitive domains consist of tandem and interspersed repeats based on nonapeptide (consensus GYYPTSP (SEQ ID NO:1) or LQQ), hexapeptide (PGQGQQ (SEQ ID NO:2) and tripeptide (GQQ) motifs, and appear to form .beta.-spiral structures based on repeated .beta.-turns (Miles et al. (1991) Proc. Natl. Acad. Sci. USA 88: 68-71). It has been proposed that these spiral structures are intrinsically elastic (by analogy with elastin) (Tatham et al. (1985) Cer. Chem. 62: 405-412; Tatham et al. (1984) FEBS Lett. 177: 205-208), although the mechanism is clearly different (Belton et al. (1994) J. Cereal Sci. 19: 115-121). The non-repetitive N- and C-terminal domains appear to be globular and form cross-links via disulfide bonds.
Elastin is comprised of a single protein containing a serial alignment of alanine-rich, lysine-containing cross-linking sequences alternating with glycine-rich hydrophobic sequences. With the entire bovine sequence known, the most striking hydrophobic sequences, both from the standpoint of length and of composition, are one that contains a polypentapeptide (PPP) and one that contains a polyhexapeptide (PHP). Elastin also contains several tetrapeptide (TP) units. As a result of work conducted by one of the present inventors, the polypentapeptide of elastin when cross-linked has been found to be elastomeric and the polyhexapeptide thereof has been found to be non-elastomeric and appears to provide a means for aligning and interlocking the chains during elastogenesis. It has also been found that the elastin polypentapeptide and polytetrapeptide are both conformation-based elastomers that develop entropic elasticity and strength on undergoing an inverse temperature transition to form a regular .beta.-turn containing dynamic structure.
Most importantly, cross-linked PPP, PTP and analogs thereof at fixed length exhibit elastomeric force development at different temperatures spanning a range of up to about 75.degree. C. depending upon several controllable variables. Moreover, these cross-linked elastomers develop near maximum elastomeric force over a relatively narrow temperature range. Thus, by synthesizing bioelastomeric materials having varying molar amounts of the constituent pentamers and tetramers together with such units modified by hexameric repeating units, and by choosing a particular solvent to support the initial viscoelastic phase, it is possible to rigorously control the temperature at which the obtained bioelastomer develops elastomeric force.
In general, the process of raising the temperature to form the above elastomeric state is an inverse temperature transition resulting in the development of a regular non-random structure, unlike typical rubbers, which utilizes, as a characteristic component, hydrophobic intramolecular interactions. The regular structure is proposed to be a .beta.-spiral, a loose water-containing helical structure with .beta.-turns as spacers between turns of the helix which provide hydrophobic contacts between helical turns and has suspended peptide segments between .beta.-turns. The elastomeric force of these various bioelastomers develops as the regular structure thereof develops. Further, a loss of regular structure by high temperature denaturation results in loss of elastomeric force. These polymers can be prepared with widely different water compositions, with a wide range of hydrophobicities, with almost any desired shape and porosity, and with a variable degree of cross-linking by selecting different amino acids for the different positions of the monomeric units and by varying the cross-linking process, e.g. chemical, photochemical, enzymatic, irradiative, used to form the final product.
These bioelastomeric polypeptides are a relatively new development that arose in the laboratories of one of the present inventors and are disclosed in a series of previously filed patents and patent applications. For example, U.S. Pat. No. 4,474,851 describes a number of tetrapeptide and pentapeptide repeating units that can be used to form a bioelastic polymer. Specific bioelastic polymers are also described in U.S. Pat. Nos. 4,132,746, 4,187,852, 4,589,882, and 4,870,055. U.S. Pat. No. 5,064,430 describes polynonapeptide bioelastomers. Bioelastic polymers are also disclosed in related patents directed to polymers containing peptide repeating units that are prepared for other purposes but which can also contain bioelastic segments in the final polymer: U.S. Pat. Nos. 4,605,413, 4,976,734, and 4,693,718,; 4,898,926, entitled "Bioelastomer Containing Tetra/Pentapeptide Units"; 4,783,523 entitled "Temperature Correlated Force and Structure Development of Elastin Polytetrapeptide"; 5,032,271, 5,085,055, and 5,255,518, entitled "Reversible Mechanochemical Engines Comprised of Bioelastomers Capable of Modulable Temperature Transitions for the Interconversion of Chemical and Mechanical Work"; 4,500,700, entitled "Elastomeric Composite Material Comprising a Polypeptide"; and 5,250,516. A number of other bioelastic materials and methods for their use are described in pending U.S. patent applications including: U.S. Ser. No. 184,873, filed Apr. 22, 1988 (now issued as U.S. Pat. No. 5,336,256); U.S. Ser. No. 07/962,608, filed Oct. 16, 1992; U.S. Ser. No. 08/187,441, filed Jan. 24, 1994; and U.S. Ser. No. 08/246,874, filed May 20, 1994 (now issued as U.S., Pat. No. 5,527,610), entitled "Elastomeric Polytetrapeptide Matrices Suitable for Preventing Adhesion of Biological Materials." All of these patents and patent applications are herein incorporated by reference, as they describe in detail bioelastomers and/or components thereof and their preparation. This information can be used in preparing and using the compositions and methods of the present invention.
While varieties of wheats have been selected for properties favorable for processing into foods, the proteins of grains are principally storage proteins held in readiness for germination; they were not selected in evolution for properties of elasticity, extensibility, etc. which would be optimal for production of breads, noodles, pastries, etc. The recent development of routine procedures for transformation of wheat (Vasil et al. (1992) Bio/Technology 10: 667-675; Weeks et al. (1993) Plant Physiology 102: 1077-1084) allows for the manipulation of wheat proteins by genetic engineering. There remains a need to improve the quality of flour doughs, especially flours produced from other, less conventional, more economically viable grains such as corn, oats, rye and millet, to improve the texture of breads, pastries and other condiments made from these agricultural products.
Relevant Literature
Belton, P.S., et al., (1994) .sup.1 H and .sup.2 H NMR relaxation studies indicate that the elasticity of the HMW subunits of glutenin is not elastin-like. J. Cereal Sci. 19: 115-121.
Miles, M. J., et al., (1991) Scanning tunnelling microscopy of a wheat gluten protein reveals details of a spiral supersecondary structure. Proc. Natl. Acad. Sci. (USA) 88: 68-71.
Moonen, J. E., et al., (1982) Use of the SDS-sedimentation test and SDS-polyacrylamide gel electrophoresis for screening breeder's samples of wheat for bread-making quality. Euphytica 31: 677-690.
Payne, P. I. (1987) Genetics of wheat storage proteins and the effect of allelic variation on bread-making quality. Plant Physiol. 38: 141-153.
Payne, P. I., et al., (1981) Correlations between the inheritance of certain high-molecular weight subunits of glutenin and bread-making quality in progenies of six crosses of bread wheat. J. Sci. Food Agric. 32: 51-60.
Pomeranz, Y and Williams, P. C. (1990) Advances in Cereal Science and Technology, 10: 492-501, Eds Pomeranz, Y., American Association of Cereal Chemists Inc., St. Paul, Minn.
Shewry, P. R., et al., (1992) The high molecular weight subunits of wheat glutenin. (Critical Review Article) J. Cereal Sci. 15: 105-120.
Tatham, A. S., et al., (1985) The .beta.-turn conformation in wheat gluten proteins: relationship to gluten elasticity. Cer. Chem. 62: 405-412.
Tatham, A. S., et al., (1984) Wheat gluten elasticity: a similar molecular basis to elastin? FEBS Lett. 177: 205-208.
Vasil, V., et al., (1992) Herbicide resistant fertile transgenic wheat plants obtained by microprojectile bombardment of regenerable embryogenic callus. Bio/Technology 10: 667-675.
Weeks, J. T., et al., (1993) Rapid production of multiple independent lines of fertile transgenic wheat. Plant Physiology 102: 1077-1084.